On the Biology of Arousal
by readALLthebooks
Summary: Sherlock discovers that there is more pleasure than he expected in a simple biological function. Warnings: lemons, slash, voyeurism.


**Hi everyone! This is the first time I have published fanfiction in years, the first time I have written for any fandom other than Harry Potter, and the first time I've tried writing a lemon. Please let me know how I did! **

**Warnings: contains lemons, mild (one-sided) slash, and voyeurism. If that's not your thing, I won't take it personally if you don't read. **

It was happening again. Normally, he was only concerned with one biological function of his penis: urination. Every once in a while, however, its other biological function would assert itself.

Today was one such day. Sherlock was aware the moment he woke of a tension in his groin. Sure enough, when he rolled out of bed, his pyjama pants were tented in the front. He let out a sound of annoyance and wrapped his dressing gown firmly around himself, disguising the bulge.

Sherlock, for all the gaps in his practical knowledge, knew what an erection was, and why men got them. He knew it was a perfectly natural biological process. That did not prevent him from feeling annoyance towards the offending appendage; while he could train his mind to filter out unnecessary data, he had not yet managed to fully master his body. He could go without food or sleep for days at a time, but he could not prevent his penis from occasionally stiffening.

He knew what would happen, too. He would ignore the problem, and eventually it would go away. But it would eventually reassert itself, usually at the worst of times. Only after several cycles of this would he give in and wrap a hand around himself, stroking himself to as fast a release as possible. Then he would not have to concern himself with that particular biological function for a while.

Something about this time was different, though. John was already reading the paper in the sitting room when Sherlock made his way in, and offered a general noise of good morning. Sherlock grunted in reply, picking up the other newspaper. Normally his discomfort would have abated by now, but instead, it had only intensified. He crossed his legs, attempting to ignore the ache that had set in.

The prospect of a new case distracted his mind from his physical discomfort, which abated for the most part as he and John crisscrossed town looking for clues. His trousers, which had felt unusually tight in the morning, were comfortable once again by the time they returned to their flat. Without that distraction, he was able to completely immerse himself in the case; he barely noticed when John told him he was going up to bed.

Hours later, the discovery was enough to impel him to run up the stairs to John's room, forgoing his usual distaste for it. He was brought up short, however, at the noises he heard emanating through the barely-open door. Curious as ever, he slowed his tread and stepped very quietly onto the landing, peering through the cracked door.

John lay on his back, legs splayed and eyes screwed shut. One hand was clutching at the sheets; the other was wrapped firmly around his erection and was moving up and down at a rapid pace.

Arousal flooded through Sherlock's body, setting off a starburst of sensation in his groin unlike anything he had ever experienced before. He hardened instantly, and barely managed to keep his hand from the resulting bulge in his trousers. He could not, however, tear his eyes away from the sight of John masturbating. Gritting his teeth against his own urgent need, he watched, mesmerized, as John's hand sped up and he began to thrust his hips into his fist. Suddenly his body tensed, and semen spurted onto his hand and belly as he let out a muffled grunt of completion.

Sherlock noted suddenly that his breathing was heightened, his pulse rapid. This was new to him—the erections, yes, those were a biological fact, and he expected them along with orgasm, but never had they accompanied mere arousal. Feeling unsteady on his feet, he made his way back down to his room and locked the door behind him.

Ignoring this was not going to work—he had to deal with it _now_. His shucked his jacket and shirt, uncharacteristically letting them fall in a heap on the floor. His fingers fumbled as he undid his belt and his flies, sucking in a breath as he brushed his straining erection. He pushed trousers and pants off his hips as he nearly collapsed onto his bed; his pants caught around one ankle, but that no longer mattered. All that mattered was relieving his now almost painful arousal.

He let out the slightest of groans as his hand finally wrapped around his engorged length. He moved it rapidly, mimicking John's pace as he saw the other man in his mind's eye. Normally, this wasn't pleasurable; it was a simple release of pressure, a biological process just like urination. Now, however, he didn't want the sensations zinging through his nerve endings to cease. He tightened his grip, biting his lip hard to stop the noises that seemed to be fighting their way out of his throat. He could not, however, prevent the rumbling of satisfaction in his chest.

The familiar sensations preceding his release began to manifest, but he found himself reluctant to end the pleasure—and he could no longer deny the pleasure he felt in this act. He tried to slow the pace of his hand, but his body was woefully out of his control now. He moaned deeply as his orgasm crested, semen forced out of his body in satisfying spurts. He kept stroking slowly as his erection subsided, finally stopping when the movement began to chafe. He laid there a moment, breathing deeply and listening to his heart pound in his ears. So this was what all the ordinary people were on about when they pursued sex.

Finally, he sat up and found a tissue with which to clean himself up. It had been a messy business, but Sherlock could not find it in him to be annoyed. On the contrary, his body still echoed with the pure pleasure of it, try though he might to tamp down the reverberations. Perhaps this business could be more than just the performing of a biological function after all.

He didn't bother to pull on his pyjamas before settling down to sleep. The case was all but solved, and the physical exertion of masturbation left him tired in both mind and body, a rare occasion. In the morning, he could go back to being Sherlock Holmes, high-functioning sociopath, accompanied by his dogged sidekick John Watson. For now, though, he allowed himself to experience the final sparks of arousal as he fell into a deep sleep.

**Thanks for reading! If you have the time and the inclination, please let me know what you think! **


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